Stability of Zinc Oxide Synthesized via Sol-Gel Method in Supercritical Water

1. Stability of zinc oxide samples prepared with sol- gel method in supercritical water AytenATEŞ CumhuriyetUniversity, Engineering Faculty, Department of Chemical Engineering, Sivas, 58140, TURKEY 3rd International ColloquiumEnergyandEnvironmentalProtection

2. ZincOxide (ZnO) a white crystalline oxide of zinc Structure Hexagonal, wurtzite crystal structure Outstandingoptical and electrical properties Uses Optoelectrictransducers, Chemicaland gas sensors, Photocatalystetc. Morphologies wires, flowers, rods, tubes, sheets, ribbons, plates, dumbbell, disks and springs Synthesisroutes Sol–gel method  Hydrothermaland solvothermal synthesis from ambient temperature and pressure up to supercritical conditions  Chemicalvapor deposition (CVD)  Thermal evaporation Spray pyrolysis

3. Use of supercritical water (SCW) with heterogeneous catalyst in biomass gasification has following advantages: • The control of phase behavior via elimination of gas- liquid and liquid- liquid mass transfer resistances, • Enhanced diffusion rate in reactions controlled by interface diffusion (fluid/particle), • Enhanced heat transfer, • Easy removal of products, • Improvement of catalyst’s lifetime by dissolution of deactivating hydrocarbons, • Improvement of solvent using a co-solvent and increased pressure.

4. Effect of supercritical water (SCW) on heterogeneous catalyst • The stability and mechanical strength of the catalysts cannot be remained in SCW as the solid- solid transformations of the conventional catalysts. • The structural change such as the aggregation of solid catalysts, phase transformation and dissolution of solid can occur in SCW. • The degree of the structural change: • The composition of catalysts • Preparation method of catalysts • Operation conditions in the reactor. The crystalline phase of zincoxide strongly affects its structural, textural, and thereby catalytic properties in SCW

5. Experimental Method Sol-gel synthesis of ZnO Characterization Temperature-Programmed Reduction (TPR) Temperature-Programmed Oxidation (TPO) Crystalinity: XRD (RigakuSmartLab). Surface area and porosity (Autosorb 1) Functional groups : FTIR (Perkin-Elmer Spectrum One) Particle size: SEM / Particle size analyzer Precursor: Zinc acetate dehydrate Chemical: Ethylene glycol, n-propyl alcohol, and glycerol, triethylene amine Preperation: Precursor, zinc acetate dehydrate, was dissolved in ethylene glycol at 150 oC for 15 min at 400 rpm. The gel was formed during aging. The gel was dissolved in glycerol and n- propanol was added to solution. After gel formation, tri ethylene amine was added to the gel and the solvent was removed by rotary evaporator at 90 oC under vacuum. Gel was dried at 80 oC and calcined at 350 oC and 900 oC.

6. Reaction Reactor:Batch reactor (Parr 4591) (volume of 100 cm3). • Mixture: 55 g/L of formaldehyde and 40 g (0.52 g/cm3) of solution ; • ZnOcatalyst: 1.0 g Reaction time and temperature: 30 minand 400 oC Gas and liquid samples were analyzed by using an on-line GC and HPLC, respectively

7. RESULTS AND DISCUSSION

8. XRD ZnO has hexagonal crystal structure and its crystal structure maintains after SCW gasification, but its crystal size is getting bigger based on SEM results

9. SEM RESULTS ZnO calcined at 350 oC has nano- sized particles ZnO-350SCW ZnO-350 ZnO calcined at 900 oC has mikro- sized particles SCW gasification leads to particle growing at both calcination temperature ZnO-900 ZnO-900-SCW

10. SEM RESULTS SCW leads to agglomeration of ZnO irrespective of calcination temperature ZnO-350 ZnO-350- HT ZnO-900 ZnO-900- HT

11. Surface area and pore size distribution Surface area of ZnO synthesized with sol- gel is higher than commercial ZnO The reaction of formaldehyde on ZnO decreased surface area due to agglomeration of crystal aMultipoint BET; b Adsorbedvolume at P/Po = 0.99’da; c DR

12. ZETA Results SCW gasification and SCW affect significantly the zeta potential of ZnO Hydrothermal treatment (HT) in SCW affects significantly the zeta potential of ZnO

13. TGA Results Coke amount ZnOcalcined at high temperature (900 oC) is more stabile than that at low temperature due to its higher crystal size based on SEM and XRD results Coking is less in the presence of ZnO- 350 than that of ZnO-900

14. Activity Results Table 1. Gas and liquid product distribution of formaldehyde on ZnO For 30 min, the highest H2 percentage is detected on ZnO-900 and highest CO2 percentage is observed on ZnO- 350. Increasing particle size increases hydrogen formation of ZnO.

15. Decomposition mechanism of formaldehyde The presence of ZnO changes decomposition mechanism of CH2O and enhanced hydrogen formation

16. Results and Discussion • Based on XRD pattern: • ZnO has hexagonal phase composition irrespective of calcination temperature • SCW and SCW gasification do not affect ZnO’s phase composition • Increasing calcination temperature of ZnO : • increasing stability based on XRD patterns • decreasing surface area and porosity • increasing crystal size

17. Presence of ZnO • changes decomposition mechanism of formaldehyde • enhances hydrogen formation • increases formaldehyde conversion

18. THANKS FINANCIAL SUPPORT The Scientific and Technological Research Council of Turkey (TUBITAK) (213 M398) and Cumhuriyet University Research Fund (M742).